فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri - Paris Observatory

The part of the → Earth's crust which underlies the
→ continents.
Continental crust is more silica-rich and thicker than
→ oceanic crust, and is on average
older. However, it is highly variable in all of these respects. The
average thickness of the continental crust is about 40km, but beneath
parts of the Andes and the Himalaya mountain ranges the crust is more
than 70 km thick. Continental crust is continuously
being eroded and turned into sediment. Some of this sediment ends up
on the ocean floor where it can be returned to the
→ Earth's mantle at
→ subduction zones.
The oldest parts of the continental crust
include some rocks that are nearly 4 billion years old.
New continental crust is produced by the destruction of
oceanic crust at subduction zones, a process that continues
today.

The rocky outermost layer of the Earth,
ranging from about 10 to 65 km in thickness.
It is distinguished from the underlying the
→ Earth's mantle layer by its
more → silicon- and → aluminium-rich
composition, lower density, and the lower velocity at which it conducts seismic energy.
It includes → continental crust (about
40 km thick) and → oceanic crust
(about 7 km thick). The crust and the
topmost layer of the mantle form the → lithosphere.
The five most abundant → chemical elements in the
Earth's crust are, in percentage by weight of the Earth's crust:
→ oxygen (O) 46%, silicon (Si) 28%, aluminium (Al) 8%,
→ iron (Fe) 5%, and → calcium
(Ca) 4%.

That part of the → Earth's crust
underling most of the Earth's surface which is covered by the oceans.
It has a remarkably uniform composition (mostly ~ 49% SiO2) and thickness
(mostly ~ 7 km). The ocean floor is the most dynamic part of the Earth's
surface. As a result, no part of the oceanic crust existing today is
more than 200 million years old, which is less than 5% of the age of
the Earth itself. New oceanic crust is constantly being generated from
the → upper mantle by sea-floor spreading at
→ mid-ocean ridges, while
other parts of the oceanic crust are being recycled back into the
mantle at subduction zones.